As a conventional solid-liquid separation unit, there is, for example, a submerged membrane unit as shown in FIG. 15. Referring to FIG. 15, in a membrane unit 21, a plurality of plate-like membrane cartridges 22, and a diffuser 23 that flows a membrane surface cleaning air from the bottom of the membrane cartridges 22, are disposed in a case 24. The case 24 consists of a membrane case 25 and a diffuser case 26, and it is designed so that the entire air dispersed from the diffuser 23 to clean membrane surface enters into the membrane case 25.
In the membrane cartridge 22, a membrane 22B is disposed on both surfaces of a supporting plate 22A made of ABS resin, and the membranes 22B are sealed, at a water sealing portion S of the periphery, to the supporting plate 22A by supersonic welding. A permeate passage is formed between the supporting plate 22A and membrane 22B, and inside of the supporting plate 22A, and a nozzle 22C connected to the permeate passage is formed at an upper edge of the supporting plate 22A.
Each membrane cartridge 22 is connected, via a tube 27 connected to the nozzel 22C, to a manifold 28. A permeate discharge pipe 29 that discharges the liquid transmitted through the membrane is connected to the manifold 28.
When a membrane unit 21 is used in an activated sludge treatment plant, the membrane unit 21 is submerged in a mixed liquor of activated sludge in an aeration tank, organic substances and nitrogen in raw water are treated with the activated sludge, under the condition that an aerated air is dispersed from an diffuser 23.
The mixed liquor of activated sludge is subjected to gravity filtration by the membrane cartridges 22 by using the water head in the aeration tank as a driving pressure (alternatively, a suction filtration may be conducted by interposing a suction pump in a permeate discharge pipe 29), and the permeate through the membrane surface of the membrane cartridges 22 is discharged, as a treated water, via the permeate discharge pipe to the outside of the tank.
At this time, the bubble swarm dispersed from the diffuser 23, and an upward current caused by the bubbling will flow a narrow watercourse (5 to 10 mm in width) between the adjacent membrane cartridges 22. Thereby, the membrane surfaces of the membrane cartridges 22 are cleaned to suppress a separation function reduction, thereby preventing a malfunction of the membrane unit 21.
Thus, since the membrane cartridge 22 is subject to an upwelling current caused by the aeration air when the membrane unit 21 is in use, the region surrounded by the water sealing portion S and the peripheral part of the membrane 22B will vibrate, and the membrane 22B might be broken at the water sealing portion S due to fatigue. Therefore, as shown in FIG. 16, it has been proposed to fix auxiliary the membrane 22B to the supporting plate 22A at a peripheral fixing portin B to be formed intermittently along the peripheral part of the membrane 22B.
One of ultrasonic welding processes is a rotary welding. In this process, as shown in FIG. 17, a membrane 22B is disposed so as to cover the surface of a supporting plate 22A, a rotary horn 31 is rotated while it presses the membrane 22B against the supporting plate 22A and, by the ultrasonic generated from the rotary horn 31, the membrane 22B is sealed to the supporting plate 22A, thereby to form a water sealing portion S and auxiliary fixing portion B.
With the rotary welding process, however, the supporting plate 22A is melted at the water sealing portion S, to make a dent in the form of a groove, as shown in FIG. 18. Since the membrane 22B is sealed to the supporting plate 22A so as to bite into the plate 22A, the membrane 22B is damaged at the time of welding, which is susceptible to a fatigue failure.
Other example of the ultrasonic welding processes is an up-down process. In this process, as shown in FIGS. 19 to 21, a linear welding allowance 32 and zonal auxiliary welding allowance 33 for forming a water sealing portion S are previously formed on the surface of a supporting plate 22A, so as to project therefrom, a membrane 22B is disposed so as to cover the welding allowance 32 and auxiliary welding allowance 33, and an up-down horn 34 is pressed against the welding allowances 32 and 33, from above the membrane 22B.
The up-down horn 34 has, on its bottom facing to the membrane 22B, a pattern 34a. The pattern 34a is formed in such a manner that 0.3-mm-high regular quadrangular pyramids are positioned from one another by a pitch of 0.6 mm.
By the ultrasonic vibration generated from the up-down horn 34, the membrane 22B is sealed to the supporting plate,22A at the welding allowances 32 and 33, to form a linear water sealing portion S and auxiliary fixing portion B.
Meanwhile, the membrane 22B is obtained by forming an organic substance on the front and rear surfaces of a nonwoven fabric serving as a substrate. Its mechanical strength is ensured by the nonwoven fabric. Therefore, in the foregoing method, the membrane 22B is locally pressed at the position corresponding to the top of the pattern 34a of the up-down horn 34, and an excessive energy is concentrated at that position. Thereby, the fabric of the nonwoven fabric is easily broken and its mechanical strength is thus impaired.
It is a primary object of the invention to provide a submerged membrane cartridge having an improved durability against aeration, as well as a method of manufacturing the same.